For previous reading, please refer to Mold Design Guidelines (Gating System and Venting Design).

Gases inside mold include not only air in cavity, but also air in runner and decomposition gases generated by plastic melt. During injection molding, these gases should be smoothly vented.
9.5.1 Harmful Effects of Insufficient Venting:
Forming burn marks, air bubbles, and seams on the surface of plastic part, making surface outline unclear; Difficulty in filling, or localized flash; In severe cases, scorch marks on the surface; Reducing mold filling speed and extending molding cycle.
9.5.2 Venting Methods
Commonly used venting methods are as follows:
(1) Opening venting grooves: Venting grooves are generally opened at the end of melt flow on parting surface of front mold, as shown in Figure 9-30. Width b = (5~8) mm and length L is about 8.0 mm ~ 10.0 mm.

 

Figure 9-30 Design of exhaust channel
1. Diversion channel 2. Exhaust channel 3. Guide groove

 

 

Depth h of venting groove varies depending on resin, mainly considering viscosity of resin and its ease of decomposition. As a general rule, for resins with low viscosity, venting groove should be shallow. For resins that are easily decomposed, venting groove area should be large. Venting groove depth for various resins can be referenced in Table 9-3.
Table 9-3 Venting Groove Depth for Various Resins

 
(2) Venting using parting surface: For parting surfaces with a certain degree of roughness, gas can be vented from parting surface. See Figure 9-31.

 

(3) Venting using ejector pins: For trapped air in the middle of molded part, ejector pins can be added. Gap between ejector pin and core can be used to vent air, or gap between ejector pins can be intentionally increased. See Figure 9-32.
(4) Venting using interlocking gaps: For modular cavities and cores, interlocking gaps can be used to vent air. See Figures 9-33 and 9-34.

 

 

(5) Adding a duct strip: For closed bone positions such as flared ribs, to improve effect of trapped air on flow, a duct strip can be added. Strip should be about 0.50 mm higher than bone position (h value). See Figure 9-35.

 

(6) Ventilation using permeable steel
Ventilable steel is a sintered alloy, a material sintered from spherical alloy particles. It has poor strength but a porous texture, allowing gas to pass through. Placing a piece of this alloy at venting location achieves venting purpose. However, diameter D of bottom vent should not be too large to prevent deformation due to cavity pressure, as shown in Figure 9-36. Because permeable steel has low thermal conductivity, it should not be overheated; otherwise, decomposition products may be generated, clogging vents.

 

Chapter 10 Mold Temperature Control
Mold temperature has a significant impact on molding quality and efficiency of plastic parts. In molds with higher temperatures, molten plastic has better fluidity, which is beneficial for filling cavity and obtaining a high-quality surface finish. However, it also prolongs curing time, making parts more prone to deformation during ejection. For crystalline plastics, it is more conducive to crystallization process, preventing dimensional changes in parts during storage and use. In molds with lower temperatures, molten plastic has difficulty filling cavity, leading to increased internal stress, a dull surface, and defects such as silver streaks and weld lines.
Different plastics have different processing properties, surface requirements and structures of various plastic parts differ. To produce plastic parts that meet quality requirements in the most efficient time, it is necessary to maintain a certain mold temperature. The more stable mold temperature, the more consistent produced plastic parts will be in terms of size, shape, and surface quality. Therefore, in addition to factors related to mold manufacturing, mold temperature is a crucial factor in controlling quality of plastic parts, and mold temperature control methods should be fully considered during mold design.

10.1.1 Principles of Mold Temperature Control
To ensure production of plastic parts with high appearance quality, dimensional stability, and minimal deformation within the most efficient time, basic principles of mold temperature control should be clearly understood during design phase.
Different plastic materials require different mold temperatures, see Section 10.1.3; Different surface qualities and structures require different mold temperatures, necessitating a targeted approach when designing temperature control system; Temperature of front mold is higher than that of rear mold, typically by about 20-30℃; Temperature of front mold requiring an EDM finish is higher than that requiring a smooth finish. When front mold needs to be circulated with hot water or hot oil, temperature difference is generally around 40℃; When actual mold temperature cannot reach required mold temperature, mold should be heated. Therefore, mold design should fully consider whether heat brought into mold by plastic material can meet mold temperature requirements; Besides being dissipated through thermal radiation and conduction, most of heat brought into mold by plastic material needs to be carried out of mold by circulating heat transfer medium. Heat in easily heat-conducting components such as beryllium copper is no exception; Mold temperature should be uniform, avoiding localized overheating or undercooling.

 

10.1.2 Mold Temperature Control Methods
Mold temperature is generally controlled by adjusting temperature of heat transfer medium, adding heat insulation plates, or heating rods. Heat transfer medium is generally water or oil, and its channels are often called cooling channels.
Lowering mold temperature is generally achieved by circulating "machine water" (around 20℃) through front mold and "cooling water" (around 4℃) through rear mold. When cooling channels cannot pass through certain parts, materials with high heat transfer efficiency should be used to transfer heat to heat transfer medium, as shown in Figure 10.1.1, or "heat pipes" should be used for localized cooling.
Raising mold temperature is generally achieved by circulating hot water or hot oil (heated by a hot water machine) into cooling channels. When a high mold temperature is required, a heat insulation plate should be added to mold panel to prevent heat loss through heat conduction.
In hot runner molds, runner plate requires a high temperature and must be heated by a heating rod. To prevent heat from runner plate from being transferred to front mold, causing cooling difficulties, contact area between runner plate and front mold should be minimized during design phase.
10.1.3 Injection Temperature and Mold Temperature of Commonly Used Rubber Materials
Table below shows commonly used rubber material injection temperature and mold temperature when surface quality of rubber part has no special requirements (i.e., a generally smooth surface). Mold temperature refers to temperature of front mold cavity.

 

10.2.1 Cooling System Design Principles
Design principle of cooling system is uniform cooling. To achieve this, there are only two points: proximity to high-heat areas and distance from low-heat areas.

 

Distance from wall of cooling channel to cavity surface should be as equal as possible, generally 15~25mm, as shown in Figure 10.2.1;
Number of cooling channels should be as large as possible and easy to process. Diameter of general water channels is selected as Æ6.0, Æ8.0, or Æ10.0, and distance between two parallel water channels is 40~60mm, as shown in Figure 10.2.1; 

 

All molded parts are required to have cooling water channels unless there is no other option. Strengthen cooling of areas where heat accumulates, such as battery pockets, horn positions, thick plastic areas, and gates. A plate, B plate, sprue plate, and gate section are determined based on specific situation; 
Reduce temperature difference between inlet and outlet. Temperature difference between inlet and outlet water will affect uniformity of mold cooling; therefore, inlet and outlet directions should be marked during design, and must be marked on mold blank during mold making. Water flow path should not be too long to prevent excessive temperature differences between inlet and outlet water;
Minimize presence of "dead water" (medium that does not participate in flow) in cooling water channels; 
Cooling water channels should be avoided at foreseeable weld lines on plastic parts; 
Ensure minimum edge distance of cooling water channels (i.e., minimum steel thickness around water holes). When channel length is less than 150mm, edge distance should be greater than 3mm; when channel length is greater than 150mm, edge distance should be greater than 5mm; 
Cooling water channel connections should be sealed with O-rings. Seal should be reliable and leak-free, see 10.2.2 for sealing structure;

 

For areas where cooling water channel layout is difficult, alternative cooling methods should be adopted, such as beryllium copper or heat pipes.
Determine location of cooling water connectors reasonably to avoid affecting mold installation and fixation.
10.2.2 Sealing Structure of O-rings
Commonly used O-ring structure is shown in Figure 10.2.3. See Section 15.5 of Chapter 15.

 

Commonly used sealing structure is shown in Figure 10.2.4. Common assembly technology requirements are listed in table: Unit (mm)

10.2.3 Cooling Examples
(1) Shallow mold cavity cooling. Front mold is shown in Figure 10.2.5, and rear mold is shown in Figure 10.2.6.

 

(2) Deep mold cavity cooling. As shown in Figure 10.2.7.

 

(3) Cooling of smaller, taller cores. Figure 10.2.8 shows a diagonally crisscrossing cooling channel; Figure 10.2.9 shows a sleeve-type cooling channel.

 

(4) For areas where cooling channels cannot be machined, heat is transferred using a thermally conductive material, as shown in Figure 10.2.10.

 

(5) Half-mold cooling. As shown in Figure 10.2.11, cooling channels are provided on half-block, recessed slots for water inlet and outlet pipes are provided on mold blank.

 

(6) Cooling of top block. As shown in Figure 10.2.12. A clearance groove is opened at interface of water outlet and water inlet pipes of top block. Size of clearance groove should meet movement space of water inlet pipe when top block is pushed out.

 

Large lifters require water circulation.

 

Water circulation design in sliders: Water circulation should be implemented in sliders whenever possible.

 

Cooling system consists of water cavities, water nozzles, water pipes, water channels, water collection blocks, and water plugs.